(1) Field of the Invention
This invention relates to cosmetic compositions in which an oily substance is made into an oil-in-water emulsion (hereinafter referred to as an O/W emulsion) with the aid of an emulsifier composed of either a combination of a basic polypeptide and a higher fatty acid, both being compounds derived from naturally occurring materials, or a salt (soap) formed from these compounds. It also relates to cosmetic compositions of the O/W emulsion type which cause no irritation to the skin, have great safety and affinity for the skin, show excellent emulsion stability and storage stability, and present an attractive appearance (fine texture and good gloss).
(2) Description of the Prior Art
It is well known in the prior art that skin cosmetics of the emulsion type, such as creams and milky lotions, are required to satisfy the following conditions:
(1) They must cause no irritation to the skin and hence have great safety for the skin. PA1 (2) They must have excellent emulsion stability and storage stability. PA1 (3) They must have good chemical stability including high resistance to hydrolysis. PA1 (4) They must present an attractive appearance from the viewpoints of texture and gloss. PA1 (5) They must have great affinity for the skin. PA1 (1) PA1 (2) from 0.5 to 60% by weight of an oily substance having no free carboxyl group, the oily substance being selected from the group consisting of higher aliphatic hydrocarbons, animal or vegetable fats and oils, ester oils, waxes, higher alcohols, and combinations thereof; and PA1 (3) from 25 to 90% by weight of water.
In order to meet these requirements, elaborate compositional designs are being made, for example, by selection of suitable emulsifiers, search for useful combinations thereof, and concomitant use of special base materials. However, it is not easy to accomplish that purpose. Especially, it is very difficult to satisfy the above-described conditions by using a single emulsifier.
For example, nonionic surface active agents of the polyoxyethylene alkyl ether type are strongly irritative to the skin and poor in emulsifying power. Nonionic surface active agents of the ester type, such as polyoxyethylene fatty acid esters, polyoxyethylene glycerol fatty acid esters, polyoxyethylene sorbitan fatty acid esters, and the like, are poor in emulsifying power and resistance to hydrolysis.
Anionic surface active agents, such as sulfuric esters of higher alcohols, alkylarylsulfonic acid salts, and higher fatty acid salts, and the like, have a strong degreasing power and cause irritation to the skin.
Cationic surface active agents and ampholytic surface active agents are also irritative to the skin.
Meanwhile, soaps made from higher fatty acids are being commonly used as emulsifiers. Typical examples thereof include triethanolamine-stearic acid, sodium hydroxide (or potassium hydroxide)-stearic acid, and borax-beeswax soaps.
However, as is often mentioned in the literature, the triethanolamine-stearic acid soap used over a long period of time is allergenic to persons having a certain constitutional disposition. The sodium hydroxide (or potassium hydroxide)-stearic acid soap involves some operational problems (e.g., difficulty in pH adjustment) and has the disadvantage of being poor in emulsifying power (e.g., the resulting emulsion tends to be broken upon exposure to high temperatures). With the borax-beeswax soap, an excess of borax is liable to induce allergy.
The present inventors have made great efforts to search for soaps (emulsifiers) derived from natural materials and characterized by the properties of inducing no or little allergy, exerting a mild action on the skin, and providing stable emulsions, and have discovered that soaps (emulsifiers) made from a basic polypeptide and a higher fatty acid can provide very stable emulsions which cause no irritation to the skin and give an agreeable feeling. The present invention is based on this discovery.